The sputtering method has been widely used conventionally as a film forming method. A prior art sputtering apparatus is shown in FIG. 7. In that apparatus, a cathode 9 and a substrate holder 2 are positioned in a vacuum container 1, and a substrate 3 to be coated is placed on the substrate holder 2. A thin-plate-shaped coating material 5, called a target, is placed on the cathode 9. The substrate holder 2 and the cathode 9 concurrently serve as electrodes. The vacuum container 1 is provided with a gas inlet conduit 4 for introducing a sputtering gas into the vacuum container 1 to sputter the coating material from the target 5.
The apparatus of FIG. 7 is operated by placing the substrate 3 on the substrate holder 2 and then reducing the pressure inside the vacuum container 1 to a value in the range of 1.times.10.sup.-1 to 1.times.10.sup.-3 Torr via the vacuum pump PU, which is connected to the vacuum container 1 via an exhaust valve 8. A suitable sputtering gas, e.g., argon, is introduced into the vacuum chamber 1 via the gas inlet conduit 4 to displace any air in the vacuum container 1. In this vacuum state, a voltage is applied between the substrate holder 2 and the cathode 9 by the power source W in order to generate an electric discharge, thereby putting the argon gas into a plasma state, and the resulting argon ions drive molecules of the coating material out of the target 5 on the cathode 9. This phenomenon of the argon ions driving out molecules of the coating material from the target is referred to as sputtering, and the sputtered coating material is deposited as a thin film on the substrate 3, which is positioned on the substrate holder 2. The film forming method by means of the sputtering phenomenon is generically called the sputtering method.
The cathode 9 can be provided with a plurality of built-in magnets for the purpose of increasing the plasma density for the improvement of a film forming rate. Such cathode is referred to as a magnetron cathode. While any suitable cathode can be employed, the sputtering method is generally implemented by the magnetron sputtering method using this magnetron cathode.
A known proposition for improving the sputtering method is illustrated in FIG. 8. For example, Japanese Patent Laid-Open (A) 8-199355 discloses the method of positioning a coil-shaped electrode 17 between the cathode 9 and the substrate holder 2 inside the vacuum container 1 of the aforementioned general sputtering apparatus, and ionizing or exciting molecules are driven out of the target 5 by sputtering, achieved by applying a high frequency voltage to this coil-shaped electrode 17. Further, in applying the high frequency voltage to the cathode 9, there is disclosed a device for avoiding a mutual interference by making the frequency of the high frequency voltage, which is applied to the coil 17 by the power source P, differ from the frequency of the high frequency voltage, which is applied to the cathode 9 by the power source W.
However, as both the sputtering of the target material from the target 5 and the depositing of the sputtered material onto the substrate 3 to form the film take place in a common chamber, the argon gas used for the sputtering is excited again by the coil 17 to which the high frequency voltage is applied, possibly causing etching of the substrate 3 by the argon gas in the film forming process. Consequently, the film forming rate can be extremely retarded and/or some of the argon can be incorporated into the film, resulting in a problem in that a film conforming to stoichiometry cannot be obtained as well as a problem in that the surface of the resulting film is rough.